GdIrSi 3 and DyIrSi 3 compounds have been investigated by magnetic susceptibility measurements and by Mössbauer spectroscopy with the 155Gd and 161Dy resonances. Both compounds were shown to order antiferromagnetically at 15.5 and 7.5 K, respectively. The 155Gd Mössbauer data show that the Gd moments are in the basal plane and allow to estimate the leading second order crystal field parameter B 0 2 for the isostructural RIrSi 3 series. It may be anticipated that the easy direction of the magnetization should be parallel to the c-axis when the Stevens factor α J is positive and in the basal when α J is negative (e.g. in DyIrSi 3). The 161Dy Mössbauer data point to the occurrence of an incommensurate modulated spin structure for DyIrSi 3. The reduced average Dy moment of 8.7μ B is attributed to crystal field effects.